1. Field of the Invention
This invention pertains to marine seismic streamer cables having a plurality of hydrophone receivers positioned therealong, each of said receivers including a pressure acceleration response element, and more specifically to the testing of said hydrophone receivers for proper polarity connection.
2. Description of the Prior Art
A marine seismic streamer cable typically comprises a plurality of sections of standard length cables, commonly 50 meters long, connected together to form a streamer, commonly in excess of 3 kilometers. Each streamer cable length contains a plurality of hydrophone receivers, which are pressure sensitive devices having an acceleration response element. These receivers can be connected into the cable in many ways. Of course, the simplest way for the hydrophones to be connected into the cable would be as individual receivers. However, this is not the most common way. The most common way for the receivers to be connected is in a standard group or array. Typically, seven separate hydrophone receivers evenly spaced apart are connected together to form an array.
The response element of the hydrophone can take on many forms, but the most common form includes a piezoelectric crystal, which responds differently to a pressure wave depending on its orientation. In a first orientation, a pressure wave with a positive onset results in an electrical voltage response signal of the same polarity. That is, a positive pressure wave produces a positive electrical voltage. In the second or opposite orientation of the response element, a pressure wave with a positive onset results in an electrical voltage response signal of the opposite polarity. That is, a positive pressure wave produces a negative electrical voltage.
When the elements are connected together, the electrical equivalent connection is as if they were connected in parallel. Assuming that all of the elements in the array are connected in the same polarity orientation, e.g., all connected for positive response, then the voltage signal outputs would be cumulative for a commonly received pressure wave. Of course, because of the spacing and the weighting of the individual receivers, a passing wave may not be received exactly simultaneously, but for the consideration at hand, the receivers are deemed to be properly oriented to produce pressure-additive results. Such a response not only produces the largest response possible for the sensitivity of the receivers, but interpreting the results will be meaningful since the measurements are treated and assumed to be from receivers connected in the same polarity.
To more fully appreciate what one or more reverse hydrophone receiver connection will do, please appreciate that while all of the properly polarity-oriented elements are producing positive response voltages, the improperly polarity-oriented elements are producing negative response voltages. Hence, instead of an additive response being produced by all of the receivers, some would be substractive. Not only would the resultant cummulative signal voltage be smaller than an all-additive cummulative signal voltage, but the total voltage output would be erroneous. Also, the erroneousness would not be predictable, because the data would not indicate which or how many of the receiver elements were reverse connected. Furthermore, the data would not appear to be in error. That is, the data would "look" proper or have the proper appearance. In summary, to have one or more individual receiver elements connected in reverse polarity is completely unacceptable.
In manufacturing a cable section, one after another of the hydrophone receivers are wired into place, along with the other internal wiring and other parts of the cable. Although there are quality control procedures established for the manufacture of a cable, the cable sections must again be tested aboard ship before the cable sections are accepted for being put into service. A seismic operating crew chief cannot abide or accept a "bad" cable section. In order to test that each hydrophone is properly connected into its array, a meter or oscilloscope is connected to the output of the cable section (or possibly the entire cable) and each hydrophone receiver is subjected to pressure by tapping (so-called "tap test") while the meter or scope is observed. An onset swinging of the meter or squiggle on the scope screen in the same direction for each of the pressurized receivers indicates that all of the receivers are commonly polarity-connected. If pressurizing any one of the receivers causes an opposite swing of the meter from the other receivers, it is reverse wired into the cable section, and, therefore, that cable section must be rejected for service. Needless to say, such testing is time-consuming and tedious, particularly considering that cable section after cable section must be tested. Such procedure also is not free from possible error because after watching a swinging needle or a squiggle on a scope screen for a long period of time, there is a certain mesmerizing that takes place resulting in the tester possibly mistaking an onset negative swing for an onset positive swing.
Therefore, it is a feature of the present invention to provide an improved polarity testing circuit for relatively quickly determining if all of the hydrophone receivers included in a cable section are connected in the same polarity.
It is another feature of the present invention to provide an improved manufacturing arrangement, wherein each hydrophone is connected with components so as to enable the assembled cable section for external testing for proper polarity connection of all hydrophone receivers.